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Thread: 120Hz hum in Mesa Boogie Nomad 45

  1. #1
    Hack with a Workbench Tone Meister's Avatar
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    120Hz hum in Mesa Boogie Nomad 45

    I have a Nomad 45 on the bench with a strong 120Hz hum coming from the speaker. Measured at the output jack to verify 120Hz. Does not respond to volume controls, all controls on front panel are off. Here are the conditions:

    All front panel controls are off
    Switching the Normal/Extreme switch, hum is greatly increased when switched to Extreme
    Pulled V1 - V4 preamp tubes one at a time in order - Hum still present until V5 phase inverter removed
    To isolate the power amp, pulled V1 - V4 and left in the V5 phase inverter tube - Hum still present
    When V5 phase inverter tube is pulled - Hum is gone
    Tried new 12AX7 in V5 - Hum still present
    Tried new quad EL84 - Hum still present
    Clipped in fresh 220uF/350V caps across each filter cap - Hum still present

    Using a test lead with an inline resistor
    --Shorted V5 pin 2 to ground - Hum diminishes substantially
    --Shorted V5 pin 7 to ground - Hum almost completely gone

    Replaced .1/400 cap in parallel with the 20K resistor at pin 2 V5a - Hum still present

    What should I check next?

    LINK TO SCHEMATIC
    https://www.schematichell.com/mesabo...ie_nomad45.pdf

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    Last edited by Tone Meister; 08-16-2019 at 11:11 PM.

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    Senior Member Pedro Vecino's Avatar
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    As a possibility, the amp uses a J175 FET to deactivate the Solo function (normal mode). Ive seen in others (Rectifiers) that when a FET fails it can induce noise. Simply removing it could you check if its related.

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    Quote Originally Posted by Pedro Vecino View Post
    As a possibility, the amp uses a J175 FET to deactivate the Solo function (normal mode). Ive seen in others (Rectifiers) that when a FET fails it can induce noise. Simply removing it could you check if its related.

    I pulled the J175 at 2PR3 which connects to the SOLO control pot but no change.

    Click image for larger version. 

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    Pulled and tested the two 220uF/300V main filter caps and they are fine.

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    Last edited by Tone Meister; 08-16-2019 at 09:39 PM.

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    OK, did a quick check of heater voltages. I measure 3.1 VAC at the molex that connects the PT to the PCB. Downstream, I measure 2.9 VAC on every preamp and power tube (pins 4 & 5) except for V1. On V1 the voltages on pins 4 & 5 are .25 VAC, so am I correct in thinking that 680/10 should be checked for a short?

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    Supporting Member The Dude's Avatar
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    Your schematic link in post #1 is dead, at least for me.

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    Yeah, well, you know, thats just, like, your opinion, man.

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    Senior Member Pedro Vecino's Avatar
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    https://schematicheaven.net/boogieam...ie_nomad45.pdf

    Can't you find 6.3VAC on the multiconnector?
    In that case, what happens if you measure it disconnected from the board?

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    Quote Originally Posted by Pedro Vecino View Post
    https://schematicheaven.net/boogieam...ie_nomad45.pdf

    Can't you find 6.3VAC on the multiconnector?
    In that case, what happens if you measure it disconnected from the board?
    I do get 6.3CAC on the multiconnector and on all tube sockets. I just realized that V1 filaments use DC and not AC as I had assumed.

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    Quote Originally Posted by The Dude View Post
    Your schematic link in post #1 is dead, at least for me.
    The file is a PDF on my computer but when uploaded it turns into a gif. I just added a link to the schematic here.

    LINK TO SCHEMATIC
    https://www.schematichell.com/mesabo...ie_nomad45.pdf

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    Check the DC filaments for hum/ripple, often the rectifier diodes for DC filaments go bad.

    Are they burnt looking/discolored?

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    Oh, and check the FX loop, does the noise change when a cord is plugged into FX return?

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    OK, pretty sure I found the issue. It was that 6K8uF/10V at V1. Gonna stick in all the preamp tubes and see what kind of signal I can pass.

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    Yep, passing a good clean signal now. Anything else to check while I have this cussed thing peeled back?

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